Add marker to skip/ignore specific fields on CRD generation

[luisdavim]

Similar request to https://github.com/kubernetes-sigs/controller-tools/issues/521 and https://groups.google.com/g/kubebuilder/c/N2A8FYNuWLA (both ended up unanswered 😞 )

In some cases we might re-use structs for CRDs and some other configuration and want to prevent certain fields form being added to the CRDs, something like a // +skip or // +ignore would be nice.